The question in the title hides a false premise. Most of the time, converting a MOV to an MP4 does not need to touch the picture at all, so there is nothing to lose. Knowing when that is true, and when it is not, is the whole trick.
MOV and MP4 are siblings, not rivals
MOV is the QuickTime File Format, which Apple published in 1991. MP4 is MPEG-4 Part 14, standardised a decade later, and its designers took QuickTime as the starting point. Both store data as a tree of “atoms” (Apple’s word) or “boxes” (the ISO word) that describe where the video frames and audio samples sit. The structure is so similar that many players will open a .mov renamed to .mp4 without complaint. The differences are in a handful of header fields and in which codecs each container officially permits.
What matters is what is inside. An iPhone or iPad records H.264 or HEVC video with AAC audio, and MP4 accepts both of those codecs. So “converting” a phone MOV to MP4 is a matter of rewriting the wrapper around the existing streams. The pixels are never decoded.
Remuxing versus re-encoding
Remux: copy the streams
A remux reads the compressed video and audio from one container and writes them, byte for byte, into another. No decoding, no encoding, no quality change, no codec support needed. It runs as fast as the file can be read.
Tinyvert does this for MOV to MP4 whenever the streams are allowed in the target. MP4 takes H.264, HEVC, AV1, VP9 and VP8 video and AAC, MP3, Opus and FLAC audio, which covers almost everything a phone, screen recorder or action camera produces. When the conversion finishes, the status line says copied. The file size barely moves, which is your proof that nothing was recompressed.
Re-encode: decode and compress again
A re-encode decodes every frame and compresses it afresh. Each pass through a lossy video encoder is a new generation: slightly softer edges, slightly muddier motion, or a bigger file to avoid both. It happens only when a remux is not possible or you ask for a change.
When a re-encode is unavoidable
HEVC on a machine without an HEVC decoder. Since iOS 11, iPhones record HEVC (H.265) by default. A remuxed HEVC MP4 is a perfectly valid file, but Windows 10 without the HEVC Video Extensions, older smart TVs, some projectors and many web players cannot decode it, so it opens with a black screen or not at all. The only fix is an H.264 copy, and that is a re-encode. On Tinyvert, Compress Video always re-encodes the video: set Quality to High and Max resolution to Original, and the output is H.264 where your browser can encode it, which Chrome, Edge and Safari on an ordinary desktop can.
Professional codecs. ProRes, DNxHD and Cineform MOVs from Final Cut, Premiere or a cinema camera are editing formats. MP4 has no place for them and browsers have no decoder for them, so Tinyvert cannot take those files. Export an H.264 or HEVC version from the editor, or use ffmpeg with libx264 as shown below.
Uncompressed PCM audio. Some cameras write LPCM audio into MOV. MP4 has no standard home for raw PCM, so the audio track is encoded to AAC while the video stream is still copied untouched.
Any change to size or quality. If you want the file smaller or the resolution lower, the frames must be decoded and re-encoded. That is a different job from conversion.
The iPhone settings that decide all this
Under Settings, Camera, Formats, there are two choices. High Efficiency records HEVC video and HEIC photos at roughly half the size of the alternative, and is required for 4K at 60 fps, 1080p at 240 fps and HDR. Most Compatible records H.264 and JPG, which play everywhere, at about double the size. Both still write .mov files, so neither setting saves you the container conversion; it only decides whether the result will need re-encoding later. The photo side of the same trade-off is covered in HEIC vs JPG.
There is a second switch under Settings, Photos, Transfer to Mac or PC. On Automatic, iOS converts HEVC to H.264 when copying over USB; on Keep Originals, it does not.
One more note for recent phones: they record HDR by default, and HDR footage remuxed to MP4 stays HDR. On a non-HDR display it can look flat or washed out. If a clip is destined for older screens, either re-encode it or turn off HDR Video under Camera, Record Video before filming.
The sensible default is to leave High Efficiency on, remux to MP4 when a site or app wants that extension, and re-encode only for a destination that genuinely cannot play HEVC.
Converting with Tinyvert
- Open MOV to MP4. It accepts
.movand.qtfiles, up to five per batch. - Leave Quality and Max resolution at their defaults. They apply only if re-encoding turns out to be necessary; the default path is a copy.
- Convert. Each row has a progress bar, and the status says copied or re-encoded when done. Rotation metadata from phones is preserved, so a portrait clip stays portrait.
- Download each file, take the batch as a ZIP, or use the Share button on a phone.
The Video Converter uses the same engine with an Output format menu for MP4, WebM, MOV and MKV, and it accepts MP4, MOV, MKV, WebM, TS and M4V. MP4 to MOV is the same remux in reverse. MP4 to WebM always re-encodes, because WebM only permits VP8, VP9 and AV1 with Opus or Vorbis.
The video tools use the WebCodecs API and run in Chrome, Edge, Brave and Opera on desktop and Android, Safari 17 or newer, and Firefox 130 or newer. The file is read into your browser’s memory and written back out on your device; nothing leaves the machine, nothing is logged, and after the first visit it works offline. Large files are limited by your device’s memory, so a long 4K clip is more comfortable on a laptop than a phone. If the MP4 needs to be smaller as well, Compress Video is the next step, and Trim Video cuts without re-encoding.
The ffmpeg equivalent
The remux is one line:
ffmpeg -i in.mov -c copy out.mp4
-c copy copies every stream without decoding. Two useful additions: -movflags +faststart moves the index to the front of the file so it starts playing in a browser before it has fully downloaded, and for HEVC from an iPhone, -tag:v hvc1 sets the tag Apple players expect, without which QuickTime refuses the file.
When a re-encode to H.264 is unavoidable:
ffmpeg -i in.mov -c:v libx264 -crf 18 -preset slow -c:a copy out.mp4
CRF 18 is visually lossless territory for most footage, and the AAC audio is copied rather than touched. Expect the H.264 file to come out larger than the HEVC it replaced.
What a remux cannot carry
Copying the streams preserves the video, the audio, the rotation flag and the duration. It does not preserve everything. Tinyvert does not carry chapter markers or subtitle tracks into the output, and camera-specific metadata such as timecode tracks and lens information is left behind too. ffmpeg with -c copy will bring mov_text subtitles and chapters along if they exist, so if those matter, that is the route to take. For most phone footage none of this applies, because there was never a subtitle track to lose.
Keep the original MOV until you have checked the MP4 plays where you need it. The conversion itself takes a few seconds for a typical file, and when the status says copied, you have lost nothing at all.